The mountains of Glacier National Park began forming 170 million years ago when ancient rocks were forced eastward up and over much younger rock strata. Known as the Lewis Overthrust, these sedimentary rocks are considered to have some of the finest fossilized examples of extremely early life found anywhere on Earth. The current shapes of the Lewis and Livingston mountain ranges and positioning and size of the lakes show the telltale evidence of massive glacial action, which carved U-shaped valleys and left behind moraines which impounded water creating lakes. Of the estimated 150 glaciers which existed in the park in the mid-19th century, only 25 active glaciers remained by 2010.[4] Scientists studying the glaciers in the park have estimated that all the glaciers may disappear by 2020 if the current climate patterns persist